Erdheim-Chester disease (ECD) is a rare histiocytic neoplasm characterized by tissue infiltration by foamy histiocytes, and chronic, uncontrolled inflammation. IgG4-related disease (IgG4-RD) is an insidiously progressive immune-mediated fibrotic disease typified by tumour-like mass formation in many affected organs. Neurologic manifestations are diverse.

Enhanced MRI of the brain and orbits showed focal pachymeningeal thickening and enhancement in the anterior cranial fossa and over the left frontal lobe with eccentric enhancement of the right optic nerve sheath. CRP was elevated (23 mmol/L to 62 mmol/L); extensive CSF and serum infectious and inflammatory investigations were unrevealing. PET body demonstrated aortitis and CT angiography suggested coronary artery vasculitis. Bone scan showed symmetric involvement of the long bones. Dural biopsy was delayed due to the Covid-19 pandemic and was completed following a protracted steroid course and a 15 mg/kg dose of cyclophosphamide. Pathology showed mixed inflammatory infiltrate and increased expression of IgG4 neutrophils. Clusters of CD68+, CD1a, and S100-negative macrophages were seen in all layers of dura. No BRAF mutation was identified.

This case demonstrates classic imaging findings of ECD including pachymeningitis, symmetric long bone involvement and aortitis. Pathology in ECD may show characteristic foamy histiocytes, that were absent in this case. This case demonstrates the challenge of biopsy interpretation following immunosuppressive and cytotoxic therapy and the difficulty of differentiating ECD from IgG4-RD.